My son was 15 when his father quit drinking. I never hid it from him because I really couldn't. His dad was there and his dad was often drunk. I think the older he got (late middle school and early high school) the more it was openly discussed, but he knew, I think, probably around the time he was 10.
I've always been honest with him about everything though. If he asks a question he gets an honest answer even if it makes me uncomfortable. It's worked for us. He and I are very close (he's away in college now and boy do I miss him) and he and RAH are close.
Your son knows. One day, when your husband is drinking, mention it and ask if he's aware of what's going on. You'll know how to handle it once you hear his answer.